If we do not want to hard-code values into our source code, we can use properties files. With the @Value annotation, Spring gives us an easy means to get properties from properties files and inject them into our code.

The @PropertySource adds a property source to Spring’s Environment. Here, the properties file is placed in the root of the classpath (and since I am using Maven default paths, that would be src/main/resources.

Declare a ConversionService to Inject Collections And Other Complex Types

We instantiate DefaultFormattingConversionService, which is “configured by default with converters and formatters appropriate for most applications”. That means that it can convert comma-separated strings to common, generic collection types, and can convert strings to dates, currencies and TimeZones, for example.

Notice that the method is declared static. ConversionService is of type BeanFactoryPostProcessor and must be instantiated very early in the Spring container lifecycle, so that its services are available when processing of annotations such as @Autowired and @Value are done. By declaring the method static, the ConversionService can be invoked without instantiating the enclosing @Configuration class. Therefore, we can use @Value (and other annotations) that make use of this converter in the same class, without running into the trouble that ConversionService is not yet instantiated.

To convert dates from a properties string to actual date objects we need an extra step: @DateTimeFormat. This annotation indicates the format of the date to Spring. In this case, we format our property strings to conform to ISO-8601 or DateTimeFormat.ISO.DATE_TIME:

Notice the #{...} that delimits the SpEL expression. It evaluates the string that comes from the properties files and parses it to a Map. To understand how this works, let’s have a look at what a literal map in a SpEL expression would look like: